Transducer assembly



United States Patent 2,990,482 TRANSDUCER ASSEMBLY Morris Kenny, 01d Bethpage, N.Y., assignor to Acoustica Associates, Inc., Glenwood Landing, LJ., N.Y., a corporation of New York Filed May 1, 1957, Ser. No. 656,372 13 Claims. (Cl. S10-9.1)

This invention relates to transducers, and more particularly to a high Q transducer assembly.

In a copending application of Robert L. Rod, Serial No. 585,889, tiled May 18, 1956, and entitled Sensing the Presence or Absence of Material, there is disclosed apparatus in which an electro acoustic transducer acts as a sensing element, and acts differently when immersed or not immersed in the material (usually but not necessarily a liquid). One form of the Rod invention utilizes the change in impedance or damping of the transducer when immersed.

The primary object of the present invention is to generally improve such a system, and more particularly the transducer used in such apparatus. A more particular object is to provide a transducer assembly with a greatly increased ratio of impedance between the immersed and non-immersed conditions. A further object is to provide an improved mounting for the transducer which will afford operation at maximum eilciency -for the intended purpose.

To accomplish the foregoing general objects, and more specic objects which will hereinafter appear, my invention resides in the transducer assembly and the elements thereof, and their relation one to another, as are hereinafter more particularly described in the following specification. The specification is accompanied by drawings in which:

FIG. 1 shows one method of using the transducer;

FIG. 2 is a schematic representation of one form of transducer embodying features of my invention;

FIG. 3 is a schematic representation of another form of transducer embodying features of my invention;

FIG. 4 is a schematic representation of still another form which includes an impedance matching transformer;

FIG. 5 is a perspective view of a transducer assembly including a tubular handle;

FIG. 6 is a section through a transducer assembly of the type shown in FIG. 2;

FIG. 7 shows some resonance curves explanatory of the operation of the invention;

FIG. 8 is a wiring diagram of one form of impedance sensitive circuit which may be used with the transducer of the present invention; and

FIG. 9 shows a modication of FIG. 8.

Referring to the drawing, and more particularly to FIG. l, a tank 12 contains a material, more specifically a liquid 14. One or more liquid level sensors may be mounted in the tank, as is indicated at 16 and 18. Each is sensitive to the presence or absence of liquid, and thus by means of cables 20 and 22 leading to suitable indicators or/ and relays for control purposes, the level or liquid in the tank may be made known or utilized. For example, with multiple fuel tanks, the lowering of liquid in a lirst tank to a point near the bottom may be used to automatically switch the fuel line to a second tank.

Referring now to FIG. 2 of the drawing, I there show a transducer assembly 24, 26, which has a length of one half wave length. 'Ihe assembly is made of an inner portion 24 and an outer portion 26 secured end-to-end at 28. In the particular form here shown, the parts 24 and 26 are cylindrical slugs, the circular end faces of which are secured together, as by means of a suitable cement. The parts are each approximately half the length of the asme p ICC

sembly, and more specifically, each part is acoustically one-quarter of a wave length long.

The outer portion 26 is made of a low loss or high Q material, specifically a metal such as stainless steel or hard aluminum. The inner portion 24 is a transducer element such as a piezo electric crystal, or a magnetostrictive transducer element. It is preferably a piezo ceramic such as barium titanate. Conductors 30 and 32 lead to electrodes at the ends of the piezo electric crystal, or coil terminals of a magnetostrictive element.

The transducer assembly 24, 26 is carried in a cylindrical housing schematically suggested at 34. The lower end 36 of this housing is connected to the transducer assembly at the junction of the parts 24 .and 26. In other words, the outer portion 26 of the assembly projects beyond the end 36 of the housing, and the projection is a quarter wave length, in consequence of which the connection is at a node or point of no vibration.

Referring now to FIG. 3 of the drawing, in this case the transducer assembly 38, 40 has an overall length of one wave length. The metal slug 40 and the transducer element or piezo element 38 are each one-half wave length long. They are secured end-to-end at 42, and conductors 44 and 46 lead to the electrodes at the ends of the ceramic element 38. The housing 48 has an end 50 located at the nodal point which is one;g,uarter wave length from the free end or sensing surface 52 of the slug. End 50 may be integral with slug 40, and soldered to cylinder 48; or housing 48, end 50, and slug 40, may be fabricated as an integral unit.

Described more generally, I may state that the transducer assembly has an acoustic length of one or more half wave lengths, that the outer and inner portions each has an acoustic length of one or more quarter wave lengths, and the outer end of the assembly projects beyond or is connected to the housing an odd number of quarter wave lengths from the free end of the outer portion, so that the housing and the assembly are connected at a point of no vibration. The inner and outer portions may differ some in physical length in order to be equal in acoustic length. They need not be equal at all. For example, if the assembly is three half waves long, the outer portion could be one and the inner portion two half waves long.

The electrodes may consist of silver plating at the ends of the ceramic element, and the leads extent to the silver plating. The ceramic is more lossy than the metal slug, and therefore has a relatively lower Q. The metal slug is characterized by good lacoustic transmission, that is, it has very little acoustic loss, and therefore has a high Q. Thus, the assembly consisting of the metal slug and ceramic element experiences a much larger change in impedance when loaded or immersed in the liquid to be sensed.

When the metal and ceramic slugs are secured together as here described, they both vibrate and function as a single transducer element, even though the source of vibration is the ceramic portion of the element. Thus, in FIG. 2, the transducer element has an overall length of a one-half wave length with maximum vibration at the ends, and =a nodal point at the middle. In FIG. 3, there is a maximum vibration at the ends and center, with two nodal points therebetween.

In general, there is minimum stress at the ends or maximum vibration points, and maximum stress at the nodal points. In my sensor, the outer and inner portions are connected at a nodal point in FIG. 2, despite the high stress, and I have found this entirely feasible, because no significant amount of power is being handled. In FIG. 3, the connection between outer and inner portions is not at a nodal point. The connection between the housing and the transducer assembly 38, 40 is at a nodal point, and therefore at a maximum stress point. This is' in contrast 3 with slugs sometimes used as a coupling between a transducer element and some medium which is to be set into vibration, for in such cases the slug is commonly made one-half wave 4long (or several one-half waves long), but not a quarter wave long.

The transducer element functions electrically like a series resonant circuit with some resistance in series. It thus has a resonance curve somewhat as illustrated at 54 in FIG. 7, with an impedance minimum at the series resonance frequency. (There is also a remote peak, not shown, corresponding to parallel resonance, not here involved.) When rising liquid reaches the lower or sensing face of the transducer element, it loads the element against vibration, and attens the resonance curve somewhat as shown at 56. Thus, there is a change in impedance, and the ratio of the two impedances is a measure of the sensitivity of the sensor for the present purpose.

With a naked crystal, this ratio might be as high as only ten-to-one. However, one requires some kind of a mount, and when the crystal is secured to a diaphragm, or is imbedded in other material, or is put in a housing, the ratio drops, and may be considerably less than tento-one due to mounting losses, However, with the present transducer assembly using the high Q slug with the ceramic, and mounting the same in the housing as shown, the ratio of impedance will be in the range of from 25 to 1 to over l0() to l, which is an improvement of more than ten times over.

One possible circuit which may be used for response to the change in impedance is illustrated in FIG. 8. This is shown using a vacuum tube 60, but in actual practice, a transistor is preferred for miniaturization. Both types of circuit are described in detail in a copending application Serial No. 65,6,293fled May 1, 1957, by Martin A. Damast entitled Electronic Apparatus for Sensing the Presence or Absence of Material which application claims the circuitry here shown. The tube 60 forms a part of an oscillator circuit having a main tuned circuit 62 in serieswith the plate of the tube. Regenerative feed back is provided by a coil 64 connected to the grid of the tube. The transducer 66 is eifectively in series with the cathode of the tube. In general, the oscillator is so adjusted that it stops oscillating when there is a high impedance at 66, and goes into oscillation where there is a low impedance at 66. The presence or absence of oscillation may be used as an indication, or may be used for purposes of control.

In the present case, the oscillator is operated at or near the bend of the characteristic curve of the tube, that is, near saturation. In consequence, the D.C. current owing in the anode circuit when there is zero r low amplitude oscillation has one value, which may be rather high. However, when oscillation is sustained, the tops of the output waves are cut off, and the average or D.C. current is depressed. This dilference in D.C. current may be used to operate a relay, the magnet 68 of which is connected in the cathode circuit as shown. This controls a movable contact 70 to open or close the relay circuit (or by using two contacts, to close either of two relay crcuits). Magnet 68 is preferably bypassed by a bypass capacitor 72. Inductor 80 provides a D.C. path for the cathode circuit.

The D.C. operating point of the oscillator is determined by a resistor 74 and capacitor 75 combined with the resistance of relay magnet 68. If desired, a resistor 76 may be added which functions as a damping resistor to broaden the resonance curve of the oscillator, and to stabilize its operation. Thus, reverting to FIG. 7, the resonance curve 78 is selected to approximately match the resonance frequency of the transducer, but broadly rather than sharply. Thus the oscillating frequency is determined by the series resonance of the transducer.

In effect, the oscillator is operated as a limiter so that limiting, the average or D.C. current drops substantially between the oscillating and non-oscillating conditions, and therefore a big change in current is available to dependably operate the relay 68.

It the transducer 66 is to be located remotely from the circuit, as is often the case, it is preferable to provide for impedance transformation at each end of the line in order to minimize capacitive effects in the line. Thus, referring to FIG. 9, the line 82 may be a coaxial cable used as a shielded conductor, and may have a length up to say feet. Impedance matching transformers are employed at the crystal 84 and at the cathode circuit 86 of the osci1- lator. These transformers are indicated at 88 and 90, and make up for the low impedance of the cable. A capacitor 87 may be provided to tune transformer 88 at the resonance frequency of the transducer 84.

The remote transformer 90 `is preferably housed in the housing of the transducer, and such an arrangement is shown in FIG. 4 which corresponds generally to the transducer of FIG. 3, except for the addition of an impedance matching transformer 90 located in housing 92, the latter being made long enough for the purpose. Thus, the conductors 94 lead downward to one winding of the transformer, while conductors 96, 98 lead from the other winding of the transformer down to the electrodes of the ceramic element 100.

For some purposes, it may be desirable to provide the sensor with a relatively long tube or handle, and such a device is shown in FIG. 5, in which the projecting portion of the slug is show-n at r102, the housing at 104, the tubular handle at 106, and the cable at 108 leading through the handle l106.

FIG. l6 shows the structural arrangement of a transducer like that which is schematically shown in FIG. 3. The metal slug which is a half wave length long is shown at 110. The barium titanate ceramic, which is also a half wave length long is shown at 112, these being cemented together `at the silver electrode 114. The end 116 of the housing may be formed integrally with the slug 110, the entire member being turned `down from a slug of larger diameter. This is secured to a cylindrical -housing wall 118, as by soldering. The opposite end is preferably flanged or stepped, and is secured to housing wall 118 by soldering. The tubular handle y12,2 passes through and is soldered to the end 120, as shown.

The ultrasonic liquid level sensor detects the presence or absence of a liquid at the probe or transducer face, and through appropriate circuitry can perform a useful function such as control valves, sound an alarm, etc. The operation is based on the change in value of the electrical input impedance of the transducer, depending on its acoustic loading. To function well, it is necessary to have a Suicient change in the electrical input impedance between the loaded and the unloaded state, that is between conditions when the transducer face is either immersed in the liquid or exposed to air.

The material selected `for the transducer may be either a piezoelectric crystal such as barium titanate or a magnetostrictive assembly such as a laminated magnetostrictive metal stack or magnetostrictive ferrite. In any case, the initial unloaded Q of the transducer should be enhanced -for optimum performance. The usual techniques of holding and encasing the transducer by thin diaphragms and acoustic isolation materials, such as cork or corprene, result in mounting losses which tend to `decrease the initial Q of the transducer, and thereby limit the usefulness of the sensor. I avoid these losses, and I maintain or increase the Q of the transducer unit, and simultaneously maintain a 4liquid-tight housing.

The ilanges 36, 50, 116 are located at a nodal point, thereby allowing the housing to be attached at a point of zero motion, without appreciable loading of the transducer. In addition, the metal slug itself when joined to the transducer has the effect of raising the overall Q of the A.C. output is Hat-topped, 0r pulse-like. With Severe 75 the sensor assembly, enhancing the loaded-to-unloaded electrical input impedance ratio. In FIGS. 3, 4 and 6 the metal slug is selected to be one-half wavelength long, at the length-mode resonant frequency of the transducer. The thin ange is either machined initially on the metal cylinder, or is attached with a suitable cement or by brazing or soldering, to the center of the metal slug. This point corresponds to a quarter-wavelength determined by the the initial transducer frequency. The overall length of the assembly is one wavelength long at the operating frequency.

In FIG. 2 the solid metal slug or cylinder is one-quarter wavelength long, attached to the transducer which also is one-quarter wavelength long, so that the overall length is one-half wavelength. The operating frequency of the assembly is at approximately one-half the Ilength mode resonant frequency of the piezo-electric element alone. The ilange 36 in the FIG. 2 design is attached or machined at the point corresponding to the piezo-electric element-metal cylinder interface. In this case, the flange 36 is again at the quarter-Wavelength point or node, and the protective housing 34 may be attached to the flange as before. One advantage of the latter design is that lfor the same operating frequency, fa much smaller unit may be fabricated. Although a metal cylinder is here used, both metal and non-metal housings may be used throughout.

The metal parts are preferably made of stainless steel or of a hard aluminum, such as that known commercially as 75S or 24 ST. The transducer element may be a quartz or Rochelle or salt crystal or it may be a ferroceramic element or a magnetostrictive device using nickel or cobalt iron, but Ithe elements here shown and preferred are made of barium titanate.

The particular element shown is designed for vibrationat a frequency of 9() kc. In such case, the slug may have ya diameter of one-half inch and a length of 1.0.7 3 inches. The housing shown in FIG. 6 has a length of two inches, and an inside diameter of -three-quarters of an inch. These values are given solely by way of example, and are not to be considered a limitation of theinvention.

It is believed that the method of making and using my improved sensor for determining the presence or absence of material, as well as -the -advantages thereof, will be apparent from the foregoing detailed description. It will also be apparent that while I have shown and described my invention in several preferred Iforms, changes may be made in the structures shown without departing from the scope of the invention, as sought to be defined in the following claims.

I claim:

l. A liquid level sensor comprising a transducer assembly in the 'form of a longitudinal vibrator having an acoustical length of n half waves where "n is an integer, said assembly being made of outer and inner portions secured end to end to constitute a unitary vibrator, each portion being acoustically n quarter waves long, the outer portion being solid metal, the inner portion being an electromechanical transducer element, a housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assembly at a region on the outer portion which is an odd number of quarter wavelengths from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cab-le extending through the housing for connection to the electromechanical transducer element.

2. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator having an acoustical length of n half waves where "11 is an integer, said assembly being made of outer and inner portions secured end to end to constitute a unitary vibrator, each portion being -acoustically "n quarter waves long, the outer portion being made of low loss, high Q material,

the inner portion being an electromechanical transducer element, a housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assembly at a region on the outer portion which is an odd number of quarter wave lengths from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, Iand an electric cable extending through the housing for connection to the electromechanical transducer element.

3. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator having an acoustical length of n half waves Where "n is an integer, said assembly being made of outer and inner portions secured end to end to constitute a unitary vibrator, each portion being acoustically n quarter waves long, the outer portion being solid metal, the inner portion being a piezo electric element, a liquid-tight housing surrounding at least the inner portion, 4the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the outer portion at a region on the outer portion which is one quarter wave length from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cable extending through the housing for connection to the piezo electric element.

4. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator having a length of one half wave, said assembly being made of outer Vand inner portions secured end to end to constitute a unitary vibrator, each portion being one quarter wave long, the outer portion being solid metal, the inner portion being an electromechanical transducer element, a housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assembly at a region on the outer portion which is one quarter wave length from the free end-of the outer portion at the junction of the outer and inner portions, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cable extending through the housing for connection to the electromechanical transducer element.

5. A liquid level sensor comprising a transducer assembly in the form of -a longitudinal vibrator having a length of onefhalf wave, said assembly being made of outer and inner portions secured end-to-end to constitute a unitary vibrator, each portion being one-quarter wave long, the outer portion being solid metal, the inner portion being a piezo electric element, a liquid tight housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one' end of the housing, said housing being connected to the assembly at a region on the outer portion which is onequarter wave length from the lfree end of the outer portion at the juncture at the outer and inner portions, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cable extending through the housing for connection to the piezo electric element.

6. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator having a length of two half waves, said assembly being made of outer and inner portions secured end to end to constitute a unitary vibrator, each portion being one half wave long, the outer portion being solid metal, the inner portion being an electromechanical transducer element, a housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assembly at a region on the outer portion which is onequarter wave length from the yfree end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cable extending through the housing for connection to the electromechanical transducer element.

7. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator having a length of two half waves, said assembly being made of outer and inner portions secured end-to-end to constitute a unitary vibrator, each portion being one-half wave long, the outer portion being solid metal, the inner portion being a piezo electric element, a liquid tight housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assembly at a region on the outer portion which is one quarter wave length from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, and an electric cable extending through the housing for connection to the piezo electric element.

8. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator, said as'- sembly being made of outer and inner portions secured end-to-end to constitute a unitary vibrator, the outer portion `being solid metal, the inner portion being an electromechanical transducer element, a liquid tight housing surrounding at least the inner portion, the free-end of the outer portion of said assembly extending beyond one end of the housing, said housing being connected to the outer portion at a region on the outer portion which is an odd number of quarter wave lengths from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, an impedance matching transformer within the housing at the other end of the transducer assembly, an electric cable extending through the housing for connection to one winding of the transformer, and connections Within the housing from the other winding of the transformer to the electromechanical transducer element.

9. A liquid level sensor comprising a transducer assembly in the form of a longitudinal vibrator an acoustical length of n half waves where "n is an integer, said assembly ybeing made of outer and inner portions secured end-to-end to constitute a unitary vibrator, each portion being acoustically quarter waves long, the outer portion being solid metal, the inner portion being a piezo electric element, a liquid tight housing sur-rounding at least the inner portion, the free end of the outer portion of said assembly extending beyond one end of the housing, the said housing being connected to the outer portion at a region on the outer portion which is one quarter wave length from the free end of the outer portion, whereby the housing and assembly are connected at a point of no vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, an impedance matching transformer within the housing at the other end of the transducer assembly, an electric cable extending through the housing for connection to one winding of the transformer, and connections within the housing from the other winding of the transformer to the piezo electric element.

10. A liquid level sensor comprising a transducer assembly having a length of two half waves, said assembly in the form of a longitudinal vibrator being made of outer and inner portions secured end to end to constitute a unitary vibrator, each portion being one half wave long, the

outer portion being solid metal, the inner portion being an electromechanical transducer element, a housing surrounding at least the inner portion, the free end of the outer portion of said assembly projecting beyond one end of the housing, said housing being connected to the assem-bly `at a region on the outer portion which is onequaiter wave length from the free end of the outer portion, whereby the housing and assembly are connected at a point of no Vibration, said outer portion constituting the sole means for supporting said inner portion within said housing, yan impedance matching transformer within the housing at the other end of the transducer assembly, an electric cable extending through the housing for connection to one winding of the transformer, and connections within the housing from the other winding of the transformer to the electromechanical transducer element.

11. Electromechanical transducer assembly adapted for liquid presence sensing and the like, comprising a composite longitudinal vibrator having an acoustical length of n half waves where n is an integer and being made of first and second portions secured end to end to constitute a unitary vibrator, each portion being acous tically n quarter wavelengths long, the iirst portion being made exclusively of a material which is an electromechanical transducer and the second portion being made exclusively of a material having a relatively lower acoustical loss characteristic than the iirst portion, a housing surrounding and spaced from at least said first portion, wall means of said housing being joined to said second portion in a peripheral region which is an odd number of quarter wavelengths from the free end of said second portion, said free end projecting outside said housing from said region, said second portion constituting the sole means for supporting said iirst portion within said housing, and electric conductor means entering said housing at another region for connection with said iirst portion.

12. Electromechanical transducer assembly adapted for liquid presence sensing and the like, comprising a composite longitudinal vibrator having an acoustical length of n half waves where n is an integer and being made of first and second portions secured end to end to constitute a unitary vibrator, each portion being acoustically n quarter wavelengths long, the iirst portion being made exclusively of a material which is an electromechanical transducer and the second portion being made exclusively of a material having a relatively lower acoustical loss characteristic than the rst portion, a tubular housing surrounding and spaced from at least said iirst portion, flange means joined to said second portion in a peripheral region which is an odd number of quarter wavelengths from the free end of said second portion, said flange means being peripherally joined to one end of said housing and constituting the sole means holding said vibrator with respect to said housing with said free end projecting from said one end thereof, andelectric conductor means entering said housing through the other end thereof for connection with said first portion.

13. Assembly according to claim 12 including a tubular extension hermetically sealed to said other end of said housing and providing a conduit for said conductor means.

References Cited in the tile of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Paiem; No. 2,999,482 June 27g 1961 Morris Kenny It is hereby certified that error appears in the above numbered patemJ requiring correction and that the said Letters Patent should read as corrected below.

Column. lg line 59,l for "or" read --f of ''3 column 3, line 44u for "Where" read when Column 57 line 29, strike out "0192, second occurrence; column 7, line 50,1 after1 vibrator insert having Signed and sealed this 3rd day of April 1962,

(SEAL) Attest:

ERNEST W SWIDER DAVID L., LADD Attesting Officer Commissioner, of Patents 

